1. Field of the Invention
The present invention is directed to a method for operating a nuclear magnetic resonance tomography apparatus, and in particular to such a method which allows separation of water signals and fat signals from each other.
2. Description of the Prior Art
U.S. Pat. No. 4,769,603 discloses a pulse sequence that is usually referred to with the acronym xe2x80x9cFISPxe2x80x9d. A specific version of this pulse sequence, wherein the gradients in all spatial directions are reset before each excitation, is referred to as xe2x80x9cTrue FISP Sequencexe2x80x9d. The relatively long transient time given this pulse sequence is reduced, as taught in U.S. Pat. No. 5,541,514, emitting a radio-frequency pulse having the flip angle xcex1/2 preceding a FISP sequence, with successive excitations having a flip angle xcex1.
The True FISP sequence exhibits excellent properties for MR imaging, namely high spatial resolution, short measuring time in the range of seconds, and high signal-to-noise ratio, particularly for tissue having liquid consistency.
The True FISP Sequence exhibits a strong T1/T2 weighting. Pure water having a ratio T1/T2=1 yields the maximum SSFP (Steady State Free Procession) signal MO/2 given FISP. MO represents the magnetization in the relaxed condition dependent on the proton density. Fat exhibits a T1/T2 ratio of 270:70 ms. Even though this ratio is clearly higher than that of pure water, fat still exhibits an extremely high signal intensity in the FISP sequence. For comparison, muscle tissue has a T1/T2 ratio of 12:1. In conventional sequences wherein a high signal intensity of fat is likewise disturbing, this problem is usually solved by fat saturation in a preparation phase. A spectrally selective radio-frequency pulse is emitted into the examination subject. In order to destroy the magnetization produced by this pulse, the radio-frequency pulse is enclosed by pulses known as spoiler gradient pulses. Such a preparation phase, however, lasts for a relatively long time (for example, 10 ms), and thus it is unsuitable for FISP, which has a typical repetition time TR=5 ms. Given the necessary lengthening to, for example, 15 milliseconds, pronounced artifacts would be obtained due to field inhomogeneities.
It is an object of the present invention to enable a separation of the fat and water signals in a FISP sequence.
The above object is achieved in accordance with the principles of the present invention in a method for operating a nuclear magnetic resonance tomography apparatus wherein radio-frequency pulses are emitted into an examination subject with a repetition time, in order to excite nuclear spins in the examination subject, nuclear resonance signals resulting from the nuclear spins are read out between the radio-frequency pulses under a readout gradient in a readout window, the nuclear resonance signals are phase-encoded before the readout window and the phase-encoding is reset before the next radio-frequency pulse, and wherein the repetition time is selected so that fat-bonded protons experience a phase rotation of approximately nxc2x7180xc2x0 due to the effect of the chemical shift compared to water-bonded protons, wherein n is an odd number.